A number of review articles describe the characterization and therapeutic relevance of the prostanoid receptors as well as the most commonly used selective agonists and antagonists: Eicosanoids; From Biotechnology to Therapeutic Applications, Folco, Samuelsson, Maclouf, and Velo eds, Plenum Press, New York, 1996, chap. 14, 137-154 and Journal of Lipid Mediators and Cell Signalling, 1996, 14, 83-87 and Prostanoid Receptors, Structure, Properties and Function, S Narumiya et al, Physiological Reviews 1999, 79(4), 1193-126.
The EP4 receptor is a 7-transmembrane receptor and its natural ligand is the prostaglandin PGE2. PGE2 also has affinity for the other EP receptors (types EP1, EP2 and EP3). The prostanoid EP4 receptor falls into a group of receptors normally associated with elevation of intracellular cyclic adenosine monophosphate (cAMP) levels. The EP4 receptor is associated with smooth muscle relaxation, intraocular pressure, pain (in particular inflammatory, neuropathic and visceral pain), inflammation, neuroprotection, lymphocyte differentiation, bone metabolic processes, allergic activities, promotion of sleep, renal regulation, gastric or enteric mucus secretion and duodenal bicarbonate secretion. Therefore EP4 receptor agonists may be useful for the treatment of pain, inflammation and other conditions associated with the EP4 receptor. The EP4 receptor also plays an important role in closure of the ductus arteriosus, vasodepression, inflammation and bone remodeling as reviewed by Narumiya in Prostaglandins & Other Lipid Mediators 2002, 68-69 557-73.
A number of publications have demonstrated that PGE2 acting through the EP4 receptor subtype, and EP4 agonists alone, can regulate inflammatory cytokines after an inflammatory stimulus. Takayama et al in the Journal of Biological Chemistry 2002, 277(46), 44147-54 showed PGE2 modulates inflammation during inflammatory diseases by suppressing macrophage derived chemokine production via the EP4 receptor. In Bioorganic & Medicinal Chemistry 2002, 10(7), 2103-2110, Maruyama et al demonstrate the selective EP4 receptor agonist (ONO-AE1-437) suppresses LPS induced TNF-α in human whole blood whilst increasing the levels of IL-10. An article from Anesthesiology, 2002, 97, 170-176 suggests that a selective EP4 receptor agonist (ONO-AE1-329) effectively inhibited mechanical and thermal hyperalgesia and inflammatory reactions in acute and chronic monoarthritis.
Two independent articles from Sakuma et al in Journal of Bone and Mineral Research 2000, 15(2), 218-227 and Miyaura et al in Journal of Biological Chemistry 2000, 275(26), 19819-23, report impaired osteoclast formation in cells cultured from EP4 receptor knock-out mice. Yoshida et al in Proceedings of the National Academy of Sciences of the United States of America 2002, 99(7), 4580-4585, by use of mice lacking each of the PGE2 receptor EP subtypes, identified EP4 as the receptor that mediates bone formation in response to PGE2 administration. They also demonstrated a selective EP4 receptor agonist (ONO-4819) consistently induces bone formation in wild type mice. Additionally, Terai et al in Bone 2005, 37(4), 555-562 have shown the presence of a selective EP4 receptor agonist (ONO-4819) enhanced the bone-inducing capacity of rhBMP-2, a therapeutic cytokine that can induce bone formation.
Further research by Larsen et al shows the effects of PGE2 on secretion in the second part of the human duodenum is mediated through the EP4 receptor (Acta. Physiol. Scand. 2005, 185, 133-140). Also, it has been shown a selective EP4 receptor agonist (ONO-AE1-329) can protect against colitis in rats (Nitta et al in Scandinavian Journal of Immunology 2002, 56(1), 66-75). Doré et al in The European Journal of Neuroscience 2005, 22(9), 2199-206 have shown that PGE2 can protect neurons against amyloid beta peptide toxicity by acting on EP2 and EP4 receptors. Furthermore Doré has demonstrated in Brain Research 2005, 1066(1-2), 71-77 that an EP4 receptor agonist (ONO-AE1-329) protects against neurotoxicity in an acute model of excitotoxicity in the brain.
Woodward et al in Journal of Lipid Mediators 1993, 6(1-3), 545-53 found intraocular pressure could be lowered using selective prostanoid agonists. Two papers in Investigative Opthalmology & Visual Science have shown the prostanoid EP4 receptor is expressed in human lens epithelial cells (Mukhopadhyay et al 1999, 40(1), 105-12), and suggest a physiological role for the prostanoid EP4 receptor in modulation of flow in the trabecular framework of the eye (Hoyng et al 1999, 40(11), 2622-6).
Compounds exhibiting EP4 receptor binding activity have been described in, for example, WO98/55468, WO00/18744, WO00/03980, WO00/15608, WO0016760, WO00/21532, EP0855389, EP0985663, WO02/50031, WO02/50032, WO02/50033, WO02/064564, WO03/103604, WO03/077910, WO03/086371, WO04/037813, WO04/067524, WO04/085430, US2004142969, WO05/021508, WO05/105733, WO05/105732, WO05/080367, WO05/037812, WO05/116010, and WO06/122403.
Derivatives of indoprofen such as [4-(1-oxo-1,3-dihydro-2H-benzo[f]isoindol-2-yl)phenyl]-2-propionic acid, sodium salt have been described by Rufer et. al. in Eur. J. Med. Chem.—Chimica Therapeutica, 1978, 13, 193.